Missing migrations directory throws unhelpful NPE

[jpolchlo]

While working on another project, I ended up spending a long time chasing down a NullPointerException inside Drift. I found the culprit being that the configured migrations directory (as indicated by the :directory key in the config map) was missing. As a new user, the NPE was unhelpful, since I had inherited my code, which I had presumed ran correctly in the past. The NPE didn't provide useful breadcrumbs.

The location in question in Drift that raised the unhelpful NPE is here: https://github.com/macourtney/drift/blob/1acfe11c292ca09f3f7cb3977cf1ebf5ac111b14/src/drift/core.clj#L114-L119

I'd imagine that a null check that throws a useful missing directory error might help someone else in the future.